Manufacture of alkyllead compounds



United States Patent MANUFACTURE or ALKYLLEAD coMroUNns Ivar T. Krohn, Royal Oak, NIicIL, assignor to Ethyl Corporation, New York, N. Y., a corporation of Betaware No Drawing. Application November 23, 1951, Serial No. 257,969

8 Claims. (Cl. 260-437) This invention relates to the manufacture of alkyllead compounds such as tetraethyllead. More particularly, the invention relates to a process for the manufacture of alkyllead compounds by alkylating lead.

Tetraethyllead is by reason of its wide use, a very important alkyllead compound and in the past has been made by the reaction of ethyl chloride with an alloy of percent sodium and 90 percent lead by weight (50 atom percent sodium). The reaction is carried out under pressure and at an elevated temperature of 75 to 90 C. in an autoclave in which the reactants are stirred or agitated. Upon completion of the reaction, excess or unreacted ethyl chloride is vented ofi, and the reaction products are discharged into water in a still. The tetraethyllead is then separated by steam distilling from the other reaction products. Other alkyllead compounds are prepared in the same way, using the appropriate alkylating agent.

While this process has been commercially successful, it has several marked disadvantages. A principal disadvantage is its restriction to the use of a sodium-lead alloy. Not only is this alloy relatively expensive, but it has to be of very critical composition, it" good yields are to be obtained. Thus, a sodiumlead alloy having a sodium content as little as /2 percent away from the 10 percent value, gives a sharply lower yield of alkyllead compound. The alloy is ordinarily made by fusing together suitable amounts of lead and sodium, followed by cooling, then grinding the mass of alloy thus produced to the particle size suitable for the subsequent alkylation and storing the ground product in an inert atmosphere till needed.

A further difiiculty of the conventional ethylation process is the fact that the reaction mixture, upon completion of the reaction, contains relatively large amounts of meallic lead which has a tendency to ball up and in general makes the extraction of the desired alkyllead product quite cumbersome. Furthermore, since this lead represents an appreciable fraction of the cost of manufacture, economy dictates that it be separately recovered, purified and re-used. The significance of this prior art difficulty will be better appreciated when it is considered that approximately three-fourths of the lead used in the prior art alkylation of the type described above must be so recovered and re-used.

A still further difiiculty with the prior commercial operations is that they have been limited to using alkyl chlorides as alkylating agents and as a a result the alkylations must be eifected under highly elevated pressures in order to carry out the reaction at the required temperatures with the alkylation agent in liquid form. For the high pressures thus necessitated the alkylation vessel must be of extremely rugged construction and is quite cumbersome in operation. In addition to the above, the prior commercial process using sodium-lead alloys converts the sodium to sodium chloride, a by-product that is substantially valueless in the form obtained.

Among the objects of the present invention is the provision of a process for making alkyllead products which avoids the above and related disadvantages.

Further objects of the present invention include the reduction in the amount of unreacted metallic lead which must be recovered by smelting operations after the preparation of alkyllead compounds. An additional object of the present invention is to provide an alkyllead preparing process that requires only extremely simple reaction vessels, the reaction zones of which are more efiiciently used. A still further object of the present invention is the provision of a novel technique for combining with the prior alkyllead forming processes to simplify the operation.

According to the present invention an alkyllead compound is prepared by heating a mixture of finely divided lead and an alkylating agent of the class of alkyl sulfates and alkyl phosphates, to a temperature between about and 200 C. until a sufficient amount of alkyllead compound is formed.

At the higher temperatures within the above range it is advisable to include in the reaction mixture a thermal stabilizer that reduces or completely prevents the thermal decomposition of the formed alkyllead compounds. Such thermal stabilizers are described and claimed in the copending Calingaert U. S. patent application Serial Number 64,259, filed December 8, 1948, now abandoned and the contents of that application are hereby incorporated in the present specification as though fully set forth herein.

it is noted that the thermal stabilizers described in the above application, include fused ring and unsaturated compounds that have boiling points at least as high as 1 C. at atmospheric pressure, and examples of which are naphthalene, styrene, crotonaldehyde, allo-ocimene, butadiene, diamylene, dipentene, trimethylethylene, divinylbenzene, cyclohexene, dicyclopentadiene, allyl iodide, chloroprene, hexachloropropylene, ethynylcyclohexanol, tiglic alcohol, 2,2'-azonaphthalene, Z-benzeneazo-l-naphthylamine, allyl isothiocyanate, anthracene, chrysene, naphthalene, alpha-methyl naphthalene, bromonaphthalene, chloronaphthalene, alpha-naphthol, betanaphthol, naphthoresorcinol, tetrahydronaphthalene, indene and naphthoquinoline.

Other stabilizers shown in the above application include hydroxy compounds or substances that readily generate such compounds. Glyceryl monostearate, glycol dilaurate, 2-nitro-2-methyl-l-propauol, phloroglucinol, resorcinol, 2,4,6-tri(dimethylaminomethyl)phenol, 2- methyl-2,4-pentanediol, ethylene brornohydrin, ethanolamine and furfuryl alcohol are examples of this type of stabilizer. To the above list there can be also added nitro" compounds, includingnitrates, nitrites, amino derivatives and azo compounds, as for example alloxan, azobenzene,

n-butyl nitrate, n-butyl nitrite, nitroethane, nitromethane, p-nitrobenzoic acid, p-nitroaniline, acetyl aminothiophene, p,p-diamino-diphenylmethane and furfuryl amine. Other stabilizers mentioned in the earlier application are halogen-containing compounds, stearyl iodide and styrene dibromide.

Among the most effective and readily available of the above stabilizers are naphthalene, styrene and closely related compounds. However the general class of unsaturated hydrocarbons, particularly aryl-substituted olefins, as Well as other unsaturated compounds, fused ring bydrocarbons, and halogen-containing compounds, nitro compounds, nitrates, nitrites, azo compounds, amino de-,

rivatives and hydroxy compounds, any of which have boiling points at least as high as 1 C. at atmospheric pressure and that are soluble in the alkyllead compound appear to be effective as thermal stabilizers.

useneral h process. he in en ion is carri out by alkylating a charge of finely divided lead in a closed and agitated reaction vessel. The alkylating agent is added thereto and the mixture then heated, preferably with agitation, to reaction temperature. The thermal stabilizer can be added before or along with the allsylating agent. Alternatively the thermal stabilizer can be withheld until the reaction mixture is heated to a temperature in the order of about 100 C. Because of the low vapor pressure of the alkylating agents employed as contrasted with those heretofore used, the reaction vessel employed for the present process can be designed for relatively moderate operating pressures.

Although in some cases a thermal stabilizer will be employed having itself an appreciable vapor pressure, they exert only a minor effect on the operating pressures used because of their solubility in the alkyllead compound produced, and because of the minor quantities employed. Upon completion of the reaction, the product mixture is cooled and then discharged to recovery operations.

The tetraalkyllead can be recovered from the product mixture in several different ways. Thus, normally, the mixture is immersed in water and the tetraalkyllead removed by passing steam through the system and vaporizing the desired product. The tetraalkyllead can also be recovered at least in part, by contacting the product mixture with a stream of water, and displacing the tetraalkyllead as a liquid product without the necessity of vaporization.

By operating in accordance with the present invention, it is possible to alkylate metallic lead directly, without forming an alloy with sodium as has been the commercial practice heretofore. T o assure success, the lead particles should be quite finely divided, that is, generally of smaller than 50 mesh particle size. The finely divided lead used can be prepared by various procedures. For example, virgin lead maybe comminuted in stages in an atmosphere of an inert gas, or in a medium of dry and easily vaporizable hydrocarbon. Such fine particles tend to form surface oxide films fairly rapidly and when such films exist, poorer results are obtained. Further, it is difficult to provide an absolutely inert atmosphere to completely prevent such a deviation from chemically pure surfaces.

It is therefore preferred to have the lead used for the alkylation finely divided just before use, and in addition kept out of contact with air from the time it is prepared and until the alkylation is under way. During the alkylationthe lead is kept fairly well covered by the alkylating agent and no additional protection is generally needed.

One highly effective technique for preparing the finely divided lead is to run a preliminary alkylation of the prior type using a sodium-lead alloy. The reaction mixture after the preliminary alkylation is a mixture of alkyllead compounds, sodium salt, excess alkylating agent, minor amounts of organic by-products, and finely divided lead. As pointed out above, the residual lead usually amounts to about three-fourths of the original lead so that a large amount is available from commercial operations. The residual lead has been found to be exceptionally active for the alkylation of the present invention, and is in fact the most effective form so far discovered. Such residual lead is preferably freed of the other products of the preliminary alkylation, in particular the alkyllead present, by extracting Such components with an organic solvent. A convenient solvent for such purpose is the alkylating agent originally used, for example ethyl chloride. Hydrocarbon fractions also form highly eflective solvents. The lead available from this source is highly acceptable, especially in that it is obtained in finely divided form, a large proportion being small enough to pass a 100 mesh c een.-

Alterna ively, th Pr l min ry l yl i mi c have its organic content distilled off, preferably by steam or by a similar partial pressure operation. Atmospheric pressures can also be used for distilling off the organic products providing an effective amount of thermal stabilizer be present in the reaction product to prevent decomposition of the alkylated compounds at the elevated temperatures necessary for such distillation.

As one specific example of the present invention, parts by weight of lead were recovered from a standard commercial alkylation of 8 to 20 mesh sodium-lead alloy having 10 percent sodium, with ethyl chloride, the recovery bein eifected by vacuum distilling off the organic material. The recovered lead contained all the sodium chloride resulting from the alkylation. To the above quantity of lead, still in the distillation vessel, was added 215 parts by weight of diethyl sulfate along with 10 parts by weight of Pblz. The reaction vessel was then closed, heated to 129 C. and maintained at this temperature for 18 hours. 1 fter this period of time, the vessel was permitted to cool, vented to release any volatile materials, and the residue slowly poured into water held in a still, and steam distilled to distill over the alkyllead compound formed during the reaction. In a typical reaction of this character the yield of tetraethyllead was 45 to 46 percent, based on the theoretical reaction:

About half the 145 parts of lead could be recovered from the reaction mixture by flushing away the lighter lead salts, and could be worked up for re-use in either the standard commercial process or in the direct alkylation.

The Pblz in the above reaction behaves as an alkylation catalyst. Results similar to that of the above example, and'with about the same yields, accompany the Cels, Bils, Csl, Znlz, Asls, Sbls, n-propyl iodide, dimethyl' sulfate or trimethyl phosphate. in each case about /2 to 5 percent of catalyst based on the weight of the lead is all that is needed. a

It will be noted that with the exception of methyl sulfate and methyl phosphate all the above catalysts are iodine compounds or free iodine. Methyl sulfate and methyl phosphate, even though containing no iodine, are particularly active in the alkylation. In fact, methyl sulfate by itself will give an effective yield when used to directly alkylate the prepared lead of the above examples, even without the presence of an iodine compound or free iodine. The addition of such catalysts, however, will cause methyl sulfate to give yields as high as 65 percent of tetramethyl lead.

The alkylation reaction of the present invention takes place at temperatures ranging from 100 to 200 C. At the higher temperatures chemical reaction takes place in appreciably less time, but the addition of from percent to 5 percent of a thermal stabilizer is desirable for the reasons given above.

In place of alkyl sulfate, alkyl phosphates may also be used for direct alkylation of finely divided lead. Here the yields run somewhat lower than with the sulfates, but yields as high as 40 percent are obtainable. Trimethyl and triethyl phosphates are preferred phosphate alkylating agents. Here again the methyl phosphate is especially active and will give a good yield even without the presence of an iodine catalyst. The alkylation with alkyl phosphate can also be eifected over the temperature range of from 100 to 200 C.

As heretofore noted, a significant practical aspect of the process is the fact that it is efliciently carried out at only d r e pre sur s in co tr st t P i0ll operations- Thus alkylation vessels can be designed for the moderate operating pressures of two or three atmospheres.

Other alkylating agents for use in accordance with the present invention are di-N-propyl sulfate, di-isobutyl sulfate, mixed sulfates such as methyl-ethyl sulfate, ethylisopropyl sulfate, tri-N-propyl phosphate, methyl-diethyl phosphate and in general any aLkyl sulfate or phosphate. The activity of these alkylating agents appears to decrease as the length of the alkyl radical increases. Thus, as indicated above, methyl sulfate and phosphate give very good yields without the assistance of the catalysts. Ethyl sulfate and ethyl phosphate give smaller but definite yields without catalysts.

instead of recovering the alkyllead compound produced in accordance with the present invention by steam distillation, as described in the specific example, the alkyllead compound can be fractionally distilled off either at atmospheric or subatrnospheric pressures. Where distillation temperatures run to about 120 C. or higher it is advisable to include in the mixture being distilled an effective amount of a thermal stabilizer for the alkyllead compound. As an additional and quite efiective recovery procedure, the reacted mixture resultant from the alkylation can be intimately contacted with Water, and as a result, the tetraalkyllead can be displace as a separate liquid phase from the other products of the alkylation.

If desired the alkylation of the present invention can be effected by passing the reactants as by means of a feed screw through a tube heated to the reaction temperature. The tube can be made long enough, or the reactant passage slow enough so that a single passage will complete the reaction and it can be carried out in a continuous manner. Alternatively the passage can be accomplished in batches, and two or more repetitive passages used for a single alkylation.

As many apparently widely diiferent embodiments of this invention may be made without departing from the spirit and scope hereof, it is to be understood that the invention is not limited to the specific embodiments hereof except as defined in the appended claims.

What is claimed is:

1. A method of preparing alkyllead compounds, which method comprises heating finely divided lead with an alkylating agent selected from the class consisting of alkyl sulfates and alltyl phosphates to a temperature between about and 200 C. and in the presence of an effective amount of a thermal stabilizer for alkyllead compounds until an appreciable amount of alkyllead compound is produced.

2. The method of claim 1 in which the thermal stabilizer is naphthalene.

3. The method of claim 1 in which the thermal stabilizer is styrene.

4. The method of claim 1 in which an efiective amount of an alkylating catalyst is present in the reaction mixture during the heating.

5. The method of claim 4 in which the alkylating cata lyst is iodine.

6. The method of claim 4 in which an iodine catalyst is present.

7. The method of claim 4 in which the alkylating catalyst is methyl sulfate.

8. A process for making alkyllead compounds which process comprising alkylating a sodium lead alloy to convert part of the lead into alkyllead compound and leave the remaining lead as a finely divided residue, and heating this residue with an alkylating agent of the class consisting of alkyl sulfates and alkyl phosphates to a temperature of about to 200 C. and in the presence of an effective amount of a thermal stabilizer for alkyllead compounds until a substantial amount of the finely divided lead is alkylated.

References Cited in the file of this patent UNITED STATES PATENTS 1,611,695 Sullivan et al Dec. 21, 1926 2,414,058 Pearsall L. Jan. 7, 1947 2,535,192 Calingaert et a1. Dec. 26, 1950 

1. A METHOD OF PREPARING ALKYLLEAD COMPOUNDS, WHICH METHOD COMPRISES HEATING FINELY DIVIDED LEAD WITH AN ALKYLATING AGENT SELECTED FROM THE CLASS CONSISTING OF ALKYL SULFATES AND ALKYL PHOSPHATES TO A TEMPERATURE BETWEEN ABOUT 100 TO 200* C. AND IN THE PRESENCE OF AN EFFECTIVE AMOUNT OF A THERMAL STABILIZER FOR ALKYLLEAD COMPOUNDS UNTIL AN APPRECIABLE AMOUNT OF ALKYLLED COMPOUND IS PRODUCED. 